Polypyrrole (PPy) is one of the most commonly studied conducting polymers due to its good stability, high conductivity, ease of preparation and non-toxicity. It has been found with wide applications in the field of chemical sensors (L. Ruangchuay et al, 2004, K. Suri et al, 2002), electromagnetic interference shielding devices (C. Y. Lee et al, 2002), electrochromic devices (O. Inganas et al, 2001) and batteries (R. P. Ramasamy et al, 2003). But chemically or electrochemically synthesized conducting polymer films have poor mechanical properties, which hinder their applications as strain sensors. This limitation may be overcome by polymerization of polypyrrole on a textile substrate. A summary of polymerisation of polypyrrole is provided in the article of “Handbook of Conductive Polymers” by G. B. Bryan et. al. Polypyrrole can be fabricated by either an electrochemical process where pyrrole is oxidized on an anode to a desired polymer film, or oxidized chemically with oxidizing agents on a substrate.
However, the flexible strain sensors exhibit low sensitivity and unsatisfying stability. D. D. Rossi et al. in Material Science Engineering, C, 7(1), 31-35 (1998); Dressware: Wearable Hardware, introduced an idea of measuring movement of body segments using conductive polymer. A sensorized glove based on the sensing fabrics of polypyrrole coated Lycra/cotton was developed, but the sensor aged severely in air and the conductivity decreased continuously. Furthermore, the saturation of the sensor occurred at a small strain of about 6%, which may be not useful. K. W. Oh et al. reported that the PPy-coated Nylon-spandex was sensitive to strain change having a deformation of 50%, but the strain sensitivity is as small as not more than 2 (J. App. Polym. Sci. 2003). X. P. Jiang et al. (J. Biomed. Mater. Res. 2002) also proposed the PPy-coated PET/Spandex can be used as a strain sensor for a large deformation of up to 50%, while the strain sensitivity is only 3. Therefore, the use of conducting polymer for fabricating high technical and smart flexible textiles is still limited. Important limitations of the use of conducting polymer include lack of conductivity stability and control of strain sensitivity.
The stability of the conductivity of polypyrrole films, prepared either electrochemically or chemically, has been discussed in numerous publications. J. C. Thieblemont et al. have published several papers including: Stability of chemically synthesized polypyrrole films (Synthetic Metals 59, (1993) 81-96), and Kinetics of Degradation of the Electrical Conductivity of Polypyrrole under Thermal Aging (Polymer Degradation and Stability 43, (1994) 293-298). In addition, V. T. Truong has published several studies including Thermal Stability of Polypyrroles (Polymer International 27, (1992) 187-195). In their findings the conductivity of polypyrrole films, powders, and coatings decreases over time according to either a diffusion controlled process or a first-order decay process. The rate of decay is related to the choice of dopant anion, the method of preparation, and the conditions of aging. The decay is significantly more rapid in the presence of air, indicating that the reaction of oxygen with the polymer backbone may be responsible for a significant portion of the conductivity loss.
Researchers have studied the stability of polypyrrole films and the control of thermal stability of the conductive films. Thermal treatment, nitrogen treatment, oxygen treatment, acid and base treatment, uses of dopants as well as voltage applied have been proved to improve the stability of the polypyrrole films. However, most of the methods described above only improve stability by sacrificing conductivity, as well as sensitivity.